Any references to methods, apparatus or documents of the prior art are not to be taken as constituting any evidence or admission that they formed, or form part of the common general knowledge.
The use of turbines/generators for extracting energy from marine flows is an industry that, unlike other renewable energies, has no proven or preferred technology. The first generation horizontal marine turbines were similar to wind turbines and were submerged within ocean or river flows. FIG. 12 illustrates this type of turbine. However, the transitioning of such windmill like devices from surface to subsea environment where the fluid density is more than 830 times greater presents ongoing difficulties including high manufacture, installation and maintenance costs which make it difficult to make the technology commercially viable.
Extraction of commercial volume marine energy requires large sized devices to sweep large areas of flow. High engineering costs are incurred as rotors are fabricated in expensive high strength composite fibre materials that are subject to axial forces acting on rotors that are typically 18 meters in diameter. A one megawatt device costs approximately $US10 million. Devices are also normally built on monopole or other foundations on the ocean floor and are therefore limited to operation on the ocean floor.
An improved marine turbine is the unidirectional cross flow turbine which offers better solutions in extracting energy from fluid flows. An example is the “Darrieus” cross flow turbine which has 2, 3, 4 or more vertical straight hydrofoil blades and is a known technology that is suitable for the marine environment. However, this type of turbine has disadvantages. One disadvantage with the turbine is that it is not self-starting. Another disadvantage is that the turbine has fluctuations in torque and can be prone to blade deflection and shaking.
To overcome some of these disadvantages, a new type of turbine was developed which is known as the “Gorlov Helical Turbine”. This turbine discarded the vertical straight turbine blades and instead uses helical aerofoil blades. This turbine has disadvantages in that the helical blades that are, connected by struts or other means to a rotatable shaft have been found to deflect, shake, fail or at least operate inefficiently in flows. FIG. 11 illustrates this type of turbine.
Another disadvantage with marine turbines is the general inability to smooth out torque fluctuations which reduces efficiency.
Another disadvantage with marine turbines is in the power take off components that convert the rotational motion of the turbine to electric power. Existing power take off modules are normally separate from the turbine.
Another disadvantage with marine turbines is high cost and manufacturing complexity in making a turbine that can withstand the axial forces encountered in water flow.
Another disadvantage with existing horizontal and cross flow turbines is in the complexity and cost to manufacture the blades.
Another disadvantage with existing horizontal and cross flow turbines for use in a marine environment is that the turbine has a central shaft which can be a flow obstruction that can decrease turbine efficiency.
Another disadvantage with marine turbines is that existing horizontal and cross flow turbines can fail in marine trials due to the harsh marine environment.
Another disadvantage with marine turbines is the difficulty in installing the turbine in position and removal of the turbine for inspection, maintenance or repair.
Another disadvantage with marine turbines is in providing a structural duct apparatus for the turbine.
Another disadvantage with marine turbines is in providing an array of turbines in such a manner that the structural duct apparatus of one turbine assists in channeling higher velocity water to an adjacent turbine.
The United Kingdom Carbon Trust (the Trust) reports that the next generation of marine energy extraction devices must demonstrate survivability in hostile marine environments and preferably have an operational capability from sea floor to sea surface within a depth range of up to 60 meters. The Trust further stated that for the industry to be successful capital, installation and operational costs must be dramatically reduced so that power supply is commercially viable.
It is an object of the present invention to provide an apparatus or method for extracting energy from a fluid flow and which may overcome at least one or more of the abovementioned disadvantages or which provides a commercial choice in the marketplace.